To control the temperature of combustion engines, to which the following description refers purely by way of example, cooling circuits are used in which a cooling fluid, normally water, is circulated by a rotary pump. As is known, the pump is operated by the combustion engine itself via a mechanical belt drive, in which a flexible belt is looped about the pulleys fitted to the drive shaft and the pump shaft. The pump is therefore operated continuously by the engine, and continuously circulates cooling fluid, even when the engine is started up and the temperature of the engine is gradually rising to the optimum running level. Prior to reaching running temperature, however, the engine does not operate at its best, and pollutant emissions are normally high, so that circulating cooling water at this time increases the time taken to reach running temperature and, consequently, the amount of pollutant emissions produced.